1. Field of the Invention
This invention relates to a sampled servo type optical disk and an apparatus for reproducing the recorded information.
2. Description of the Related Background Art
As a format for an optical disk serving as an optical recording medium, there is known a sampled servo type in which the servo area and data area are cyclically and alternately arranged in the reading direction of information. Also, for a sampled servo optical disk of the kind, there is known a narrow track type which makes it possible to perform a double density recording by narrowing each pitch of the recording tracks.
FIG. 1 is a view showing the known structure of the servo area in an optical disk of such a sampled servo type.
As shown in FIG. 1, the servo area of such optical disk comprises wobble pits P.sub.W1 and P.sub.W2 for tracking servo; a mirror section D for synchronization detection and focusing servo; clock pits P.sub.C for reproducing clock phase detection; and track identifying pits P.sub.DET for identifying the track positions (even-numbered track or odd-numbered track). Each of these wobble pits P.sub.W1 and P.sub.W2, mirror section D, clock pits P.sub.C, and track identifying pits P.sub.DET is aligned and formed in the radial direction of the disk.
However, if there is a rotational fluctuation in the rotational speed of the spindle motor of a recording apparatus for the master disk which records each of the pits in the servo area, each of the clock pits P.sub.C to be formed on the respective tracks is affected by such a rotational fluctuation, and is eventually recorded in a position slightly displaced from the ideal recording position. For example, as shown in FIG. 2, the positions of the center points of the clock pits P.sub.C formed respectively on the tracks B and D adjacent to the track C are displaced by .DELTA.t.sub.2 and .DELTA.t.sub.1 with respect to the position of the center point t of the clock pit P.sub.C on the track C.
Here, if it is assumed that there are no cross talks at all from the adjacent tracks when the information is read, a waveform of a read-out signal obtainable from the clock pit on the track B in FIG. 2 presents the waveform A shown in FIG. 3. In such a waveform A, its peak value p is at the position of center point t +.DELTA.t.sub.2 of the clock pit. Also, a waveform of a read-out signal obtainable from the clock pit on the track C in FIG. 2 presents the waveform B shown in FIG. 3. In such a waveform B, its peak value q is at the position of center point t of the clock pit. Also, a waveform of a read-out signal obtainable from the clock pit on the track D in FIG. 2 presents the waveform C shown in FIG. 3. In such a waveform C, its peak value r is at the position of center point t+.DELTA.t.sub.1 of the clock pit.
However, in the narrow track type, there tends to be the influence of cross talks from the adjacent tracks to the extend that the track pitch is narrowed. In other words, the read-out signal obtainable when the clock pit formed on the track C is read is affected by the cross talks from the clock pits formed on the tracks B and D. In this case, a waveform of a read-out signal obtained when the clock pit on the track C is read presents a waveform D produced by synthesizing the waveform A, waveform B, and waveform C shown in FIG. 3. The waveform of the read-out signal obtained from the clock pit recorded on the track C should inevitably be time shifted by t.sub.z as compared with a case where no cross talk is present at all.
Therefore, in the reproducing clock whose phase is corrected on the basis of the read-out signal obtained from the clock pit which has been time shifted, jitters are inevitably generated, thus causing a problem that the recorded information cannot be reproduced in a good precision.